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Chapter 7 Microbial Growth and Growth control

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Title: Chapter 7 Microbial Growth and Growth control


1
Chapter 7 Microbial Growth and Growth control
2
Chapter outline
7.1 Overview of Cell Growth 7.2 Population Growth
7.3 Measurement of Growth 7.4 Continuous
Culture The Chemostat 7.5 Effect of Environment
on Growth 7.6 Growth Control 7.7 Viral
Control 7.8 Fungal Control
3
Concepts
  • Microbial growth is defined as an increase in
    cellular constituents and may result in an
    increase in a microorganisms size, population
    number, or both.
  • A wide variety of techniques can be used to
    study microbial growth by following changes in
    the total cell number, the population of viable
    microorganisms, or the cell mass.
  • Solid objects can be sterilized by physical
    agents such as heat and radiation Liquids and
    gases are sterilized by heat, radiation, and
    filtration through the proper filter.
  • A knowledge of methods used for microbial
    control is essential for personal and public
    safety.

4

7.1 Overview of microbial growth
  • The bacterial cell is a synthetic machine that is
    able to duplicate itself. The processes involve
    as many as 2000 chemical reactions of a wide
    variety of types
  • Energy transformations.
  • Biosynthesis of small molecules--the building,
    blocks of macromolecules-as well as the various
    cofactors and coenzymes needed for enzymatic
    reactions.

5
Growth definition
Growth may be generally defined as a steady
increase in all of the chemical components of an
organism. Growth usually results in an increase
in the size of a cell and frequently results in
cell division.
6
Cell life cycle in Eukaryotic cells
G1 Primary growth phase of the cell
during which cell enlargement occurs, a gap phase
separating cell growth from replication of the
genome
S Phase in which replication of the
genome occurs
G2 Phase in which the cell prepares for
separation of the replicated genomes, this phase
includes synthesis of microtubules and
condensation of DNA to form coherent chromosomes,
a gap phase separating chromosome replication
from miosis.
M Phase called miosis during which the
microtubular apparatus is associated and
subsequently used to pull apart the sister
chromosomes.
7
Eukaryotic cell Prokaryotic cell
G1 S G2 M
G1 R D
8
Binary fision
  • Most bacterial cells reproduce asexually by
    binary fision, a process in which a cell divides
    to produce two nearly equal-sized progeny cells.
  • Three processes
  • Increase in cell size (cell elongation)
  • DNA replication
  • Cell division

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7.2 Population Growth
  • Growth is defined as an increase in the number of
    microbial cells in a population.
  • Growth rate is the change in cell number or cell
    mass per unit time.
  • The interval for the formation of two cells from
    one is called a generation.
  • The time required for this to occur is called the
    generation time.

11
Exponential Growth
  • A growth experiment beginning with a
    single cell having a doubling time of 30 min is
    presented. This pattern of population increase,
    where the number of cells doubles during each
    unit time period, is referred to as exponential
    growth.

12
Calculating Generation Times
  • N N02n N final cell number. N0 initial
    cell number, and n number of generations.
  • n ( log N0 - log N ) /log 2 ( log N0 - log
    N ) / 0.301 3.3 (log N- log N0) .
  • The generation time g of the cell population is
    calculated as t / n, where t is simply the hours
    or minutes of exponential growth.

13
Growth Cycle of Populations
A typical growth curve for a population of cells
can be divided into several distinct phases
called the lag phase, exponential phase,
stationary phase, and death phase.
14
Growth curve of bacteria
  • Lag Phase
  • Exponential Phase
  • Stationary Phase
  • Death Phase

15
Lag Phase
When a microbial population is inoculated
into a fresh medium, growth usually does not
begin immediately but only after a period of time
called the lag phase, which may be brief or
extended depending on the history of the culture
and growth conditions.
This happens because for growth to occur
in a particular culture medium the cells must
have a complete complement of enzymes for
synthesis of the essential metabolites not
present in that medium.
16
Exponential Phase
It is a consequence of the fact that each cell
divides to form two cells, each of which also
divides to form two more cells, and so on.
Most unicellular microorganisms grow
exponentially, but rates of exponential growth
vary greatly. In general, prokaryotes grow faster
than eukaryotic microorganisms
17
Stationary Phase
  • If a single bacterium continued to grow
    exponentially for 48 hr, produce a population
    that weighed about 4000 times the weight of
    Earth! This is particularly impressive because a
    single bacterial cell weighs only about
    one-trillionth (10- l2) of a gram.

An essential nutrient of the culture medium is
used up or some waste product of the organism
builds up in the medium to an inhibitory level
and exponential growth ceases,or both.
18
Death Phase
  • If incubation continues after a population
    reaches the stationary phase, the cells may
    remain alive and continue to metabolize, but they
    may also die. If the latter occurs, the
    population is said to be in the death phase.

19
7.3 Measurement of Growth
  • Population growth is measured by following
    changes in the number of cells or weight of cell
    mass.

20
Total Cell Count
  • The number of cells in a population can
    be measured by counting a sample under the
    microscope, either on samples dried on slides or
    on samples in liquid. With liquid samples,
    special counting chambers must be used.

Direct microsopic counting procedure using the
Petroff-Hausser counting chamber.
21
Viable Counting method
  • The usual practice, which is the most valid
    statistically, is to count colonies only on
    plates that have between 30 and 300 colonies. To
    make a 10-fold (10-1) dilution, one can mix 0.5
    ml of sample with 4.5 ml of diluent, or 1.0 ml
    sample with 9.0 ml diluent.

22
The usual way to perform a viable count is to
determine the number of cells in the sample
capable of forming colonies on a suitable agar
medium. There are two ways of performing a plate
count the spread plate method and the pour plate
method.In either case the sample must usually be
diluted before plating.
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Sources of Error in Plate Counting
  • The number of colonies obtained in a viable count
    depends not only on the inoculum size but also on
    the suitability of the culture medium and the
    incubation conditions used It also depends on
    the length of incubation.
  • The length of incubation.
  • Some tiny colonies may be missed during the
    counting.
  • The incubation conditions (medium, temperature,
    time).
  • Key dilutions must be prepared.

25
Turbidimetric Measurements of Cell Number
  • A cell suspension looks cloudy (turbid) to
    the eye because cells scatter light passing
    through the suspension. The more cells present,
    the more light scattered and hence the more
    turbid the suspension.

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7.4 Continuous Culture The Chemostat
A continuous culture is essentially a flow
system of constant volume to which medium is
added continuously and from which continuous
removal of any overflow can occur. Once such a
system is in equilibrium, cell number and
nutrient status remain constant, and the system
is said to be in steady state.
28
The Chemostat
  • The most common type of continuous culture
    device used is a chemostat, which permits control
    of both the population density and the growth
    rate of the culture. Both parameters can be set
    by the experimenter.

29
Continuous culture of microorganisms
Chemostat used for continuous cultures. Rate of
growth can be controlled either by controlling
the rate at which new medium enters the growth
chamber or by limiting a required growth factor
in the medium.
30
Relationship among nutrient concentration, growth
rate(solid line),and growth yield(dashed line)in
a batch culture(closed system). At low nutrient
concentrations both growth rate and growth yield
are affected.
31
Steady-state relationships in the chemostat. The
dilution rate is determined from the flow rate
and the volume of the culture vessel.Thus,with a
vessel of 1000ml and a flow rate through the
vessel of 500ml/hr, the dilution rate would be
0.5hr-1. Note that at high dilution rates,
growth cannot balance dilution, and the
population washes out. Note also that although
the population density remains constant during
steady state, the growth rate (doubling time)can
vary over a wide range. Thus, the experimenter
can obtain populations with widely varying growth
rates without affecting population density.
32
Experimental Uses of the Chemostat
  • Even over rather wide ranges, any desired growth
    rate can be obtained in the chemostat by simply
    varying the dilution rate.
  • A practical advantage to the chemostat is that a
    population may be maintained in the exponential
    growth phase for long periods, for days and even
    weeks. The experimenter using the chemostat can
    have such cells available at any time.

33
7.5 Effect of Environment on Growth
  • The activities of microorganisms are greatly
    affected by the chemical and physical conditions
    of their environments. Understanding
    environmental in fluences helps us to explain the
    distribution of microorganisms in nature and
    makes it possible for us to devise methods for
    controlling microbial activities and destroying
    undesirable organisms.
  • Four main factors
  • Temperature, pH, water availability, and
    oxygen.

34
Effect of temperature on bacterial growth rate
Bacteria grow over a range of temperatures they
do not reproduce below the minimum growth
temperattire nor above the maximum growth
temperature. Within the temperature growth range
there is an optimum growth temperature at which
bacterial reproduction is fastest.
35
  • Enzymes exhibit a Q10 so that within a
    suitable temperature range the rate of enzyme
    activity doubles for every 10' C rise in
    temperature.

36
Microorganisms are classified as psychrophiles,
mesophiles.thermophiles, and extremethemophiles
based on their optimal growth temperature.
37
Effect of oxygen concentration on the growth of
various bacteria in tubes of solid medium.
(a) Obligate aerobes-growth occurs only in the
short distance to which the oxygen diffuses into
the medium.
(b) Facultative anaerobes growth is best near the
surface, where oxygen is available, but occurs
throughout the tube.
(e) Microaerophiles, aerobic organisms that do
not tolerate atmospheric concentrations of
oxygen-growth occurs only in a narrow band of
optimal oxygen concentration
(c) Obligate anaerobes-oxygen is toxic, and there
is no growth near the surface.
(d) Aerotolerant anaerobes-growth occurs evenly
throughout the tube but is not better at the
surface because the organisms do not use oxygen.
38
Bacteria Neutral
condition Fungi Acidic
condition Actinomycetes Alkaline
condition
39
Water activity
The water activity of a solution is 1/100 the
relative humidity of the solution (when expressed
as a percent), or it is equivalent to the ratio
of the solution's vapor pressure to that of pure
water.
40
aw P solution / P water
  • Approximate lower aw limits for microbial growth
  • 0.90 1.00 for most bacteria, most algae and
    some fungi as Basidiomycetes,Mucor, Rhizopus.
  • 0.75 for Halobacterium, Aspergillus
  • 0.60 for some saccharomyces species

41
Water activity
  • If the concentration of solutes, such as
    sodium chloride, is higher in the surrounding
    medium (hypertonic), then water tends to leave
    the cell. The cell membrane shrinks away from
    the cell wall (an action called plasmolysis), and
    cell growth is inhibited.

Normal cell
Plasmolyzed cell
42
7.6 Growth Control
Definitions
Sterilization the process of destroying all
forms of microbial life on an object or in a
material. Disinfection the process of
destroying vegetative pathogens but not necessary
endospores. Antisepsis chemical disinfection of
skin, mucous membranes or other living tissues
43
Prokaryote Control
  • Many antibiotics active against prokaryotes are
    also produced by prokaryotes.
  • Include the aminoglycosides, the macrolides, and
    the tetracyclines. Many of these antibiotics have
    major clinical applications.
  • The tetracyclines and the ß-lactam antibiotics
    are the two most important groups of antibiotics
    in the medical field.

44
Aminoglycoside antibiotics
  • Contain amino sugars bonded by glycosidic linkage
    to other amino sugars .
  • Inhibiting protein synthesis at the 30S subunit
    of the ribosome.
  • Including streptomycin and its relatives,
    kanamycin, gentamicin, and neomycin , are used
    clinically against gram-negative Bacteria.

Structure of kanamycin, an aminoglycoside
antibiotic, an aminoglycoside antibiotic.The site
of modification by an N-acetyltransferase,
encoded by a resistance plasmid, is indicated.
45
Macrolide antibiotics
  • Contain large lactone rings connected to sugar
    moieties.
  • Include erythromycin, oleandomycin, spiramycin,
    and tylosin.
  • Acts as a protein synthesis inhibitor at the
    level of the 50S subunit of the ribosome.

Structure of erythromycin, a typical macrolide
antibiotic.
46
Tetracyclines
  • The tetracyclines inhibiting almost all
    gram-positive and gram-negative Bacteria.
  • Tetracycline is a protein synthesis inhibitor.
  • It interferes with 30S ribosomal subunit
    function.

Structure of tetracycline and important
derivatives.
47
7.7 Viral Control
  • Viruses actually use the host cell machinery to
    perform their metabolic functions. Therefore,
    most attempts at chemical control of viruses
    result in toxicity for the host.
  • Several agents are more toxic for the virus than
    the host, and there are a few agents produced by
    the host that specifically target viruses.
  • There are several classes of chemotherapeutic
    agents that have been shown to be clinically
    effective in controlling viral replication.

48
Antiviral Chemotherapeutic Agents
  • The most successful and commonly used agents for
    antiviral chemotherapy are the nucleoside
    analogs. The first compound to gain universal
    acceptance in this category was
    azidothymidine(AZT), Azidothymidine is chemically
    related to thymidine but is a dideoxy derivative,
    AZT inhibits HIV. Because the normal host cell
    function of DNA replication is targeted, these
    drugs almost always exhibit some level of host
    toxicity.
  • Several other chemicals work at the level of
    viral polymerase.
  • A relatively novel class of antiviral drugs are
    the protease inhibitors.

49
Antiviral chemotherapeutic compounds
50
Interferon
  • Interferons are antiviral substances produced by
    many animal cells in response to infection by
    certain viruses. They are low-molecular-weight
    proteins (17,000 MW) that prevent viral
    multiplication in normal cells by stimulating the
    production of antiviral proteins.
  • There are three molecular types, IFN-a, produced
    by leukocytes IFN-ß, produced by fibroblnsts
    and IFN-?, produced by immune cells known as
    lymphocyles.

51
7.8 Fungal Control
  • Since fungi are Eukarya, much of their cellular
    machinery is the same as in higher animals and
    humans, and so chemotherapeutic agents that
    affect metabolic pathways in fungi often affect
    corresponding pathways in host cells. This
    results in drug toxicity in higher animals. Thus,
    many antifungal drugs can be used only for
    topical (surface) applications.
  • Some drugs are selectively toxic for fungi. Drugs
    for fungal treatment are becoming increasingly
    important as fungal infections in
    immunosuppressed individuals become more
    prevalent.

52
Ergosterol Inhibitors
  • Two major groups of antifungal compounds
    work by interacting with ergosterol or inhibiting
    its synthesis. In most fungi, ergosterol replaces
    the cholesterol component found in higher
    eukaryotic cell membranes.

Sites of action of some antifungal
chemotherapeutic agents. Because fungi are
eukaryotic cells, antibacterial antibiotics are
generally ineffective.
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Other Antifungal Agents
  • A number of other antifungal drugs interfere with
    fungus-specific structures and functions. Several
    drugs such as the polyoxins inhibit cell wall
    synthesis by interfering with chitin
    biosynthesis.
  • Other drugs inhibit folate biosynthesis,
    interfere with DNA topology during replication,
    or, like griseofulvin, disrupt microtubule
    aggregation during mitosis.
  • Some very effective antifungal drugs also have
    other biological applications. For example,
    vincristine, vinblastin, and toxol are effective
    antifungal agents and have known anticancer
    properties.

55
REVIEW QUESTIONS
  1. What is the difference between the growth rate
    constant (k) of an organism and its generation
    time (g)?
  2. Describe the growth cycle of a population of
    bacterial cells from the time this population is
    first inoculated into fresh medium. How can the
    growth pattern differ when it is measured by
    total count or by viable count?

56
  1. Describe briefly the process by which a single
    cell develops into a visible colony on an agar
    plate. With this explanation as a background,
    describe the principle behind the viable count
    method.
  2. How can a chemostat regulate growth rate and cell
    numbers independently?
  3. List three chemical classes of compatible solutes
    produced by various microorganisms. List at least
    two things they all have in common.

57
  • Concerning the pH of the environment and of the
    cell, in what ways are acidophiles and
    alkaliphiles different? In what ways are they
    similar?
  • What tests would you perform to decide whether a
    chemical agent could be used as an antiseptic? As
    a disinfectant? Some chemicals serve both
    purposes. Describe the properties of such a
    chemical and give an example.
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